1. Technical Field
The present invention relates to an optical element and a method for manufacturing the same, a liquid crystal device, and an electronic apparatus.
2. Related Art
A wire grid polarization element (hereinafter, simply referred to as “polarization element” in some cases) is known as one of optical elements having a polarization separation function. The element has a number of conductive micro-wires arranged at a pitch smaller than the wavelength of light. The element also has a property of reflecting a light component (TE) having a polarization axis parallel to the micro-wires and transmitting a light component (TM) having a polarization axis perpendicular to the micro-wires among light components of incident light.
When such a wire grid polarization element is built in a transflective liquid crystal device, a wire grid and a scattering layer are layered in an area corresponding to a reflective display region in one pixel region. The surface of the wire grid has ridges and valleys to reflect and scatter light for achieving favorable display characteristics in a reflective display. This is because that the wire grid having a flat surface shows extremely high luminance in a specular direction only. This results in lowering the luminance in a viewing direction, making it difficult to see images. Refer to JP-T-2002-520677.
Such a wire grid described above is formed on an inside surface of a resin layer. The inside surface corresponding to a reflective display region has ridges and valleys. The wire grid is formed by the following manner. First, as shown in FIGS. 22A and 22B, a metal film 512 having a light reflection property such as aluminum is formed on a concave-convex surface 511A of a resin film 511 by a vacuum film forming process. The metal film 512 is formed with a thickness of about 0.1 μm and its surface has ridges and valleys (concave-convex shape) tracing the surface shape of the resin film 511. A pitch P between convex portions 512a is about 10 μm.
Then, a resist film 513 having photosensitivity is formed on the metal film 512 showing such a concave-convex shape. The resist film 513 is subjected to two-beam interference exposure and development so as to form a resist pattern. The metal film 512 is dry-etched with the resist pattern. Accordingly, a great number of micro-wires are formed, thereby providing the wire grid.
Here, as shown in FIG. 22A, a height H of asperity of a concave-convex surface 512A of the metal film 512 is about 1 μm. When a resist is applied to the concave-convex surface 512A by spin coating, as shown in FIG. 23, most of the resist collects inside a concave portion 512b while the resist is hardly applied on the convex portion 512a. As a result, the resist film 513 cannot be formed with a uniform thickness. In this case, an exposure amount with respect to the resist film 513 differs from place by place (the convex portion 512a and the concave portion 512b), thereby not providing a resist pattern having a uniform shape. That is, even if the resist film 513 shown in FIG. 23 is exposed, a desired resist pattern cannot be formed on the convex portion 512a because the resist film 513 has not been applied on the convex portion 512a, as shown in FIG. 24.
FIG. 25 shows the metal film 512 having been etched by using the resist pattern shown in FIG. 24 as a mask. As can be seen from FIG. 25, the metal film 512 on a concave portion 511b of the resin film 511 is favorably etched whereas the metal film 512 on a convex portion 511a is mostly removed, thereby exposing the concave-convex surface 511A of the resin film 511. Therefore, it is obviously difficult to form a wire grid on the concave-convex surface 511A of the resin film 511 due to the problems in processes as described above.
This structure also has a problem from a point of view of the performance of a liquid crystal device. As shown in FIGS. 22A and 22B, the height H of the asperity of the resin film 511 (metal film 512) is about 1 μm. This concave-convex shape causes variations in the thickness of a liquid crystal layer. Typically, the liquid crystal layer is designed with a thickness of about 5 μm. Thus, the thickness of the liquid crystal layer varies by about 20 percent in a plane. This variation causes deterioration in contrast of images.
A conceivable structure is shown in FIGS. 26A, 26B, and 27. FIG. 27 is a sectional view of FIGS. 26A and 26B. As shown in FIG. 26A, a diffraction function layer 614 is disposed on a substrate 6. The diffraction function layer 614 is a structure having a larger period than a wavelength of visible light. On the surface of the diffraction function layer 614, a grid 615 as shown in FIG. 26B is disposed. This structure can reduce a height g (difference in height) of a step 616 of the diffraction function layer 614 to about 0.1 μm as shown in FIG. 27. That is, the height of about 1 μm in related art can be successfully reduced to about one tenth. This reduction drastically reduces the thickness variation caused in resist application when the grid 615 is formed. As a result, a resist pattern having a uniform thickness can be achieved. In addition, the thickness variation of a liquid crystal layer can also be reduced, preventing a contrast from lowering.
The conceivable structure described above can drastically reduce the surface step (difference in height of the step 616) of the diffraction function layer 614 on which the grid 615 is formed as compared to the structure in related art. However, in forming the grid 615, a forming defect of the grid 615 may occur because a resist pattern R is incompletely formed in the vicinity of the step 616.
FIG. 28 shows a resist that is spin-coated on the diffraction function layer 614 (difference in height of the step 616 is about 0.1 μm). Then, a resist film 617, which is the resist formed in the above, is subjected to two-beam interference exposure, providing a resist pattern shown in FIG. 29.
FIG. 29 is a sectional view showing a part of the resist film in FIG. 28 after the exposure. In FIG. 29, the resist pattern R seems to be formed roughly on an entire surface of the diffraction function layer 614. However, some portions in the vicinity of the step 616 may not be completely fixed due to lack of an exposure amount at the bottom of the resist film 617.
It is conceivable that this occurs because of an intensity distribution in a plane produced by a phase modulation of exposure light due to a shape of steps on the resist surface. Since the conceivable structure can drastically reduce the difference in height as compared to the one in related art, it is favorable as long as the resist is flatly applied so as to fill in the difference in height when the resist is applied. However, if the resist surface has the difference in height, portions that are not completely fixed will occur.